Conventional pallets are used in spacecraft for transporting payloads into outer space, for example to a space station or for transporting payloads back to earth from a spacecraft or station. For this purpose the payloads are either rigidly or releasibly secured to the pallets. The pallets in turn are mounted in the spacecraft generally in a so-called space shuttle. The mounting of the pallet in the cargo bay of the shuttle is usually accomplished by a yoke or bail and a number of journal bearing pins for securing the pallet to the bail.
One type of conventional pallet particularly constructed for the above purpose has an upper milled structure and a lower milled structure. These two milled structures are connected to each other by spacers and tension bolts. Sheet metal members for transmitting shearing forces effective on the pallet are installed between the upper and lower milled structure. These shearing force transmitting sheet metal members are either screwed or riveted to the milled structures.
Another conventional type of such pallets includes longitudinal carrier beams usually produced as extrusion sections, and interconnected by short cross-beam sections. Payload securing elements are integrated into the longitudinal extrusion sections or beams.
A third known pallet has a structure made of a multitude of angular members, sheet metal sections, and other semi-finished components which are secured to one another by screws and rivets. Such a structure requires auxiliary plates that are screwed to the pallet between the beams for securing the payload and for increasing the stiffness or rigidity of the entire pallet.
The just described conventional pallets have a relatively small rigidity or stiffness and a construction or layout that does not always provide the required strength at specific points where it is needed. Conventional pallets are relatively heavy which reduces the payload. Conventional pallets have a low resonance frequency which is undesirable for space flight missions where resonance vibrations must be avoided. Yet another drawback is seen in the fact that the complicated pallet construction contributes to the manufacturing and assembly costs at least in certain instances. For example, the machining of milled structures out of thick solid plates is expensive as well as time consuming, particularly where more than 95% of the material must be removed by milling. Furthermore, a milling operation, especially where large volume portions are removed, can result in stress concentration locations within the pallet, whereby the possible payload is substantially reduced for such a pallet. The connection of structural components by means of spacers and tension bolts is not only difficult and time consuming, it also provides a pallet, the structure of which is not ideally adapted to the load requirements to which such a pallet is exposed. On the other hand, the riveting or screwing of shear take-up plates on both sides of a milled structure having a U-cross-sectional configuration is not advantageous in view of the denting characteristics of such shear plates. Further, it is difficult to secure the shear plates to U-sectional stock, especially to the open side of such stock.
European Patent Publication EP 0,199,513 A2 discloses a lightweight structural pallet which avoids at least some of the above outlined drawbacks. The known pallet disclosed in the just mentioned European Patent Publication is constructed for air freight carrying and is made completely of aluminum, whereby the known pallet has a relatively low mass (weight) and provides an increased mechanical strength. However, even this improved air freight pallet leaves room for improvement.